The present invention relates to the gas fuel supplied to a gas turbine, and more particularly to a method and system that utilizes a steam injection system to modulate the properties of the gas fuel.
Due to the continuous surge in natural gas demand, the supply of pipeline natural gas has become unable to satisfy the demand for gas fuel. To meet this shortfall, gas turbines operators are beginning to use liquefied natural gas (LNG) as an alternative gas fuel source. The increase in LNG usage has raised the issue of interchangeability between pipeline supplied natural gas and LNG, when burned in a gas turbine combustion system.
LNG typically has a higher heating value (HHV) and Wobbe Number (described below), than natural gas. LNG may be diluted with an inert such as nitrogen to reduce the Wobbe Number, to that of the pipeline natural gas. However, this process increases the costs and lowers the competitiveness of LNG. Consequently gas suppliers are aiming to expand the allowable gas interchangeability tariff. However, this leads to wider variations in properties of the gas fuel supplied to operators of gas turbines, which may significantly impact the combustion characteristics of a gas turbine combustion system, as described below.
Before the impact on combustion characteristics is discussed, the following two fuel parameters should be defined: the incoming gas Wobbe Number (WN) and the Modified Wobbe Index (MWI) of the gas supplied to the turbine. The WN is defined as:
                    WN        =                  HHV                      SG                                              [        1        ]            where:                HHV is the higher heating value of the gas fuel; and        SG is the specific gravity of the gas fuel or gas fuel and steam mixture relative to airThe WN is used as an interchangeability index to permit gas fuels of various heating values to be utilized in the same combustion system without changing hardware. Temperature is not included in this equation for WN because gas is typically delivered at approximately ground temperature with little variation throughout the year.The MWI is defined as:        
                    MWI        =                  LHV                                    (                              SG                ×                                  (                                      460                    +                                          T                      g                                                        )                                                                                        [        2        ]            where:                LHV is the lower heating value of the gas fuel or gas fuel and steam mixture; and        Tg is the gas fuel or gas fuel and steam mixture temperature in degrees F.        
MWI more accurately measures the energy delivered through a fuel nozzle at a given pressure ratio than WN. This distinction between MWI and WN becomes very important when gas fuel is heated before delivery to the gas turbine.
Driven by market demands, power plant operators could purchase gas from different local pipelines during different times of the day. If the gas between two pipelines exhibits significantly different compositions and heating values, then a “null point” could be created near a power plant located between the natural gas pipeline and the pipeline holding LNG. A power plant situated between the “null point” could experience daily multiple shifts in gas composition. The sudden increase in variation of gas properties across the pipelines due to LNG usage, significantly affects the operability of the combustion system. Since it would be impractical to tune the combustion system to account for this variation, operation beyond the capability of the combustor could result, leading to increased combustion dynamics and operation outside of emissions compliance.
As discussed, some gas suppliers inject inerts, such as N2 or CO2, to control the gas heating value and WN if the gas supply has an unacceptably high WN.
There are a few problems with the currently known systems. The quantity of the inerts injected is minimized because it reduces the HHV value of the gas on a Btu basis. The cost of delivered gas may increase when inerts are utilized to lower the HHV.
For the foregoing reasons, there is a need for a method and system for reducing the HHV of a gas fuel. The method and system should permit adjustment of the MWI over a wide range without the need for significant temperature adjustment of the gas fuel. The method and system should provide a diluent for reducing the LHV and the resulting MWI. The method and system should not require an additional fuel separator and a fuel superheater. The method and system should not significantly increase the cost of delivered gas per unit of energy when compared to aforementioned systems.